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Sampl ing High-Pressure Gases

This paper describes fundamental relationships

between air or gas volume, pressure, and flow, andhow these relationships affect particle counting. Also,a simple plumbing technique is offered as analternative method for occasionally sampling high-pressure inert gases.

Sample Volume, Pressure, and Flow Rate

When a sample volume of media (air or gas) movesfrom a high-pressure zone to a low-pressure zone,the movement causes changes in pressure and flow.

An accurate particle counter must account for thesechanges because they can rapidly occur. Mass flowcontrollers or pressure sensors inside the particle

counter can compensate for changes in volume andpressure, and provide constant flow.

Volume, pressure, and flow have different units ofmeasure, so they are distinct and offer uniquechallenges for particle counters. Volume is theamount of space occupied by a three-dimensionalobject and is measured in cubic units (e.g., cubic feet).Pressure is the amount of force per unit area and istypically measured in pounds-per-square-inch (e.g.,PSI). Flow is the movement of media during a specifictime period and is commonly measured in cubic-feet-per-minute (CFM) or liters-per-minute (LPM).

Arguably, pressure has the most significant effect on

the particle counters flow rate and the sample volumeavailable for analysis. Particle counter design requiresan understanding of Boyles Gas Law (shown below)that states: as pressure increases, volume decreases.

P1V1= P2V2

whereP = pressure and V = volume

Applying the equation above, we can understand thata cubic foot of air at sea level is not equal to a cubicfoot of air at 5000 feet. The higher altitude andcorrespondingly lower atmospheric pressure allow thevolume of air to expand.

When compared to sea level pressure, atmosphericpressure at 5000 feet is about 18% less. As anexample, if a volume of air at sea level contains 10particles, the same volume at 5000 feet will containabout 6 particles. This result occurs because theparticles disperse through the expanded volume thathas increased about 189% at the higher altitude. If aflow rate control system is not in place, a particlecounter adjusted to provide one CFM of air at sealevel, will provide significantly inaccurate particlecounting data at much higher altitudes.

Flow Rate Control

Volumetric corrections for ambient air pressure arenecessary, so particle counters usually have flow-ratecontrols that change pump speed or allow excessairflow to bypass the sampling chamber.

These controls are adjustable through softwaresettings or mechanical adjustments, or areautomatically regulated based upon ambient pressureconditions. Each method monitors the ambientpressure, which is equal pressure on both sides of thepump, and approximates the pumps correct flow rate.

So, if a particle counter is adjusted for sea-levelpressure (14.69 psi) and then is taken to an altitude of

5000 feet (12.23 psi), the particle counter will requirealtitude control adjustments.

NOTE: Particle Measuring Systems LasairIII

Particle Counters (Figure 1) will automaticallyrecognize an over-pressure condition andreject the sample as invalid data. Otherparticle counters ignore an over-pressureenvironment and allow the sample to proceed,which provides inaccurate particle data.

Sampling gases at pressures greater than sea level,such as 40100 psi, are more challenging.Connecting pressurized air or gas to a particlecounter can defeat its flow-metering system. HighPressure Diffusers (HPDs) were developed to correctthis problem. HPDs (Figure 2) reduce the gaspressure to approximate sea level pressures. Theyoperate by allowing some of the excess air/gas toventor diffuseto the atmosphere. Then, theparticle counters flow-metering system can functionas designed.

Figure 1. Lasair III Particle Counter

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Figure 2. High Pressure Diffuser

Although the HPD is a good solution for periodicallysampling high-pressure gases, some customers mayuse an economical pressure-release valve. Thepressure-release T valve easily connects to mosthigh-pressure air/gas lines and provides a solution for

customers who infrequently sample their air/gas lines.However, this option is more costly in terms of air/gasconsumption because the valve discharges highergas volumes than normally required for monitoring. If

high-pressure air/gas sampling is more frequent, anHPD may offset the costs of air/gas loss.

Conclusion

Altitude, or more specifically, atmospheric pressure,

has a significant effect on particle counters and mustbe considered when monitoring particle contamination.The particle counter should provide some method tocompensate for different altitudes. If sampling high-pressure gases is required, the user has severaloptions: a dedicated gas particle counter; an HPD; ora pressure-release T valve.

Dedicated gas counters were not discussed becausethey are beyond the scope of this article. Decidingwhich option to choose is simply a matter ofdetermining the particle size requirements, frequencyof sampling, and financial costs of gas consumption.

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Author: Steven KochevarSenior Applications EngineerParticle Measuring Systems

Lasair is a registered trademark of Particle Measuring Systems.

2012 Particle Measuring Systems. All rights reserved.

Reproduction or translation of any part of this work without the permission of the copyright owner is unlawful. Requests for permission or

further information should be addressed to Particle Measuring Systems, Inc. at 1-800-238-1801

5/10/2012 AN74